I barely know anything about the old arcade games and how they work. I guess it would be better if you provided what system you are trying it on.
The only thing I can think of is if you are playing it on the PC, your keyboard might not be able to handle more than 3 buttons pressed at the same time.
I tried to do a stage fatality in that arena and it won't work. The other stage fatalities work though. I have Midway Treasures Extended Play for PSP.
You're not trying to use the character specific stage fatalities for the Dead Pool, are you? You need to hold LP and LK then perform a normal uppercut to knock an opponent into the acid. It's done the same for way for each character.
Oh. Ok I'll try it.